The mutant rat "fatty" shows extreme obesity of juvenile onset. A very large strain (45tL) has a slowly progressing corpulence of adult onset. Both of these obesities develop on any type of normal diet and can be studied in both the pre-obese and the late phase of fattening. Including non-fattening controls, and rats which will fatten abnormally when fed a calorically concentrated high carbohydrate diet, a total of 5 genetically different rats are available for study. The fatty shows skeletal stunting which motivates the investigation of a possible relative deficit of growth hormone. The role of insulin in obesity will also be explored. Observations to be made include: short-term cyclic changes in blood and tissue indications of fat synthesis, transport, mobilization, in relation to meal eating (high and low fat meals) and fasting and as affected by hormone administration: long-term distribution of a restricted food supply between fat stores and non-fat tissue, as affected by diet and hormones; study of the pre-obese and very early fattening stage, especially levels of circulating insulin, free glycerol, albumin, lipoproteins; relative size and number of fat cells in various depots.